Metal structures in high temperature environments such as in gas turbines may be coated with a protective ceramic insulating layer called a thermal barrier coating (TBC). Various processes and thermal barrier compositions have been used, but usually have been limited to layers less than 2 mm thick due to thermal expansion differences between the coating and the metal. This limits the amount of protection provided by these coatings, and leads to high thermal gradients in the coating, which can cause spalling. Differential thermal expansion can crack the coating and weaken the bond with the protected substrate material.
Other approaches to adhering ceramic coatings to metal substrates include the use of metal foams or feltmetals. U.S. Pat. No. 5,605,046 (Liang) and others use fibrous metallic layers brazed to metal substrates and used as a compliant layer for ceramic TBCs. Improvements, such as U.S. Pat. No. 6,499,943 (Beeck, et al) focus on improving the temperature capability of the compliant metallic interlayer.
Other approaches include: graded composition coatings from metal-to-ceramic (e.g., U.S. Pat. No. 5,320,909) metal wire meshes bonded to the metallic substrate (e.g., U.S. Pat. No. 6,280,584 and U.S. Pat. No. 6,264,766); metallic protrusions integrally cast with the substrate (e.g., U.S. Pat. No. 6,720,087 & 6,251,526); metallic foam interlayers (U.S. Pat. No. 6,544,003); and metal honeycombs (U.S. Pat. No. 6,235,370).
The problem with all these solutions is the temperature limitation of the metallic interlayers. For porous or thin-walled metal structures, oxidation resistance is severely compromised by high surface area and rapid depletion of protective oxide forming elements. Thus, the compliant member becomes the temperature limiting feature of such designs. For applications where high heat flux and/or temperatures necessitate the use of ceramic thermal barrier coatings, improvements over these state-of-the-art solutions is desired.
Thus, there has been a long-standing need for thicker coatings with improved bonding and durability on metal structures for high temperature environments.
The present invention provides a high temperature, oxidation-resistant compliant layer between a structural metal substrate and an insulating ceramic coating. The compliant interlayer comprises a fiber-reinforced ceramic composite structure which is integrally tied to both metallic and ceramic coating members and is arranged in such a manner as to provide compliance for differential thermal expansion.